Specific artifacts that users are likely to encounter in their JWST data are provided in this series of articles. In them, you will find notes on how to best run individual pipeline steps offline to optimize data quality for different kinds of science, and summaries of some common workarounds to existing problems.

Despite the generally high calibration quality of JWST data, there are nonetheless a series of known issues with these data products that science users should be aware of. These range from detector artifacts (which cannot be fixed) to reference file and pipeline software problems (which can be improved over time) to optional offline processing improvements that can be made for some science cases.

The articles are structured as follows:

• This page highlights known issues affecting all JWST observing modes.
• Instrument-specific pages highlight known issues affecting MIRI, NIRCam, NIRISS, and NIRSpec data.
• For each instrument-specific page, child pages discuss known issues affecting specific instrument modes (e.g., NIRcam imaging, MIRI MRS, etc.).
• An overview of frequently-encountered issues and workarounds across all JWST observing modes are compiled in a high-level summary page.

Please also refer to JWST Calibration Status for an overview of the general calibration quality of JWST data (photometric calibration, wavelength calibration, etc.).

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Artifacts

Cosmic ray shower and snowball artifacts

Cosmic rays typically produce small artifacts in detector data that are easily handled by the JWST pipeline, but certain kinds of cosmic rays can produce large artifacts that cover hundreds or thousands of detector pixels. See Shower and Snowball Artifacts

1/f noise

JWST detectors exhibit different types of frame-to-frame noise produced by the readout electronics. One of particular interest for observations of faint targets or time-series observations (see also JWST Time-Series Observations Noise Sources) is the so called 1/f noise in near-IR data. The most obvious visual feature of this noise is an apparent "banding" of the background pixels in a frame/group. Removing the impact of 1/f noise on the data is quite complex given the stochastic nature of the effect (every frame will have its unique signature—however, they will all share similar statistical properties). While the reference pixel correction helps diminish its effect in some cases, in general, extra steps may need to be taken in an attempt to correct for it.

At present, there is no common way of treating this effect across all three near-IR detectors. For information on 1/f correction in NIRCam data, see NIRCam 1/f Noise Removal Methods; for NIRSpec see NIRSpec Known Issues.

Note that this 1/f noise is primarily a product of the micro-electronic SIDECAR ASICs in use for the NIR detectors. The full size electronics in use by the MIRI instrument generally have more stable voltage and current supplies, and the MIR detectors therefore do not see significant 1/f noise.

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Pipeline notes

Incorrect world coordinates

See also: JWST Pointing Performance, JWST Attitude Control Subsystem

When observing without target acquisition, the placement of a given scientific target can be limited by multiple factors, including both the JWST blind pointing uncertainty and any uncertainty in the target coordinates themselves.

Even when using target acquisition to place a source precisely at the intended location within a given instrument however, the world coordinates embedded into the final data products by the JWST pipeline are subject to an observatory-wide WCS accuracy error. The magnitude of this error varies by field and by instrument, and is typically largest for the MIRI MRS (1-σ radial accuracy about 0.3", with known examples over 1.0"). These errors are driven by a combination of uncertainties in the guide star catalog, misidentified guide stars during observations, and uncertainty in the spacecraft roll angle as measured from the star trackers.

This in turn can cause problems with mosaic tile alignment, registration against data from other observatories, and spectral extraction from fixed target locations using sky coordinates.

Mitigations for this problem depend on the instrument mode in use. Imaging modes can typically use the tweakreg step to re-register coordinates against known sources within the field of view, while IFU observations can benefit from auto-centroiding to find the appropriate location for point source spectral extraction.  See the individual instrument and mode known issues articles for further details.

In the long term, work is ongoing to improve guiding performance.

Pointing jitter or drift

See also: JWST Pointing Performance, JWST Communications Subsystem, JWST Attitude Control Subsystem

The JWST calibration pipeline typically assumes that the pointing of the telescope is perfect and constant over the course of an observation. In reality, the initial target placement may be imperfect, and over many hours the pointing may be subject to drifts, jitter, or jumps. The high gain antenna repointing, for example, has been observed during commissioning to have some impact on the telescope pointing when it is moved. 

Such drift may have particular consequences for time-series observations. 

For time-series imaging, the automated photometry step in calwebb_tso3 assumes a fixed location of the target in the field of view. If this location changes significantly over the course of the time-series observation, the photometry measurement will be performed off-center and return inaccurate results. 

For time-series spectroscopy, the wavelength calibration and photometric calibration factors are applied to the data assuming perfect and constant target placement in the field. Any drifting behavior or jitter, therefore, might introduce systematic noise in the final time series. There is currently no mitigation for this in the calibration pipeline; any pointing changes must therefore be measured and corrected manually in the time series. A suggested workaround is to skip the photometric calibration step by modifying the execution parameters for the calwebb_spec2 stage since for many science needs it is the relative variations that are of most interest. Further pipeline work in this area is planned.

In general, time-series observations may find it useful to monitor the JWST guide star data which can be retrieved via MAST, and use it to study any significant pointing aberrations that might be impacting a given TSO. Details on how to perform this data retrieval can be found in JWST Time-Series Observations Noise Sources.

Time stamps across the JWST detectors

The detectors used by JWST instruments do not read all their pixels at the same time. While this is typically irrelevant for most science cases, it can be important for high-cadence time-series observations.

For the near-infrared detectors, the reading process controlled by the SIDECAR ASIC sequentially reads pixels in the "fast-read" direction. In NIRISS/SOSS frames, the reading process starts in one of the corner pixels and moves along the fast-read direction, reading one pixel at a time at a cadence of 10 microseconds per pixel. When all the pixels in a given "fast-read" column are read, the detector has a wait time of 120 microseconds before moving to the next one. This process is repeated until the entire subarray is read, after which extra wait time (which depends on the exact subarray configuration) is spent before considering the frame "read". Note that for this NIRISS/SOSS example, this means not all columns are read at the same times, but instead, some are read earlier than the rest. In practice, for NIRISS/SOSS, this means that the longer wavelengths are effectively read by the detector at different times than the shorter wavelengths—the difference being about 5 s in total per frame. This is currently not accounted for in the pipeline. Users should study whether this offset might impact their science cases, and consider making corrections to their observations based on this so every wavelength has the correct time stamp associated with it.

The time constants may be different for each instrument, and dependent on the subarray and read mode. For more detail see the NIRCAM, NIRISS, NIRSpec, and MIRI detector readout pages.

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Summary of common issues and workarounds

The tables below provide summaries of some of the most likely issues for users to encounter when working with JWST data in general, along with any workarounds if available. Note that greyed-out issues have been retired, and are fixed as of the indicated pipeline build. For a more extensive list see the individual known issues articles for different instruments and observing modes.

General issues

General issues for time-series observations

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